Most prior art methods for commercially preparing silicon nitride powder by the direct nitriding of metallic silicon powder rely on fixed bed batchwise systems. However, the fixed bed batchwise systems yield silicon nitride powder products of varying quality. Often the percentage of .alpha. phase varies from batch to batch. Even a single batch shows local variations in the temperature distribution in the furnace and the progress of nitriding reaction, also resulting in silicon nitride powder of varying quality. Variations in quality of silicon nitride powder become larger as the batch scale is increased. Another problem of the fixed bed batchwise systems is difficulty to automate raw material supply and product withdrawal steps, leaving a risk of contamination. Therefore, scale-up of the fixed bed batchwise system for commercial mass production invites many problems including greater variations in quality of silicon nitride powder products, heavy duty operations, greater labor inputs, and long heating and cooling periods.
A variety of methods have been proposed to address the problems. For example, it is known to use a vertical furnace (see Japanese Patent Application Kokai No. 151311/1983), a pusher type tunnel furnace (see Japanese Patent Application Kokai No. 186406/1985), a rotary kiln (see Japanese Patent Application Kokai No. 266305/1986), and a fluidized bed (see Japanese Patent Application Kokai No. 97110/1986).
However, few such methods can solve the problems of silicon nitride powder quality and production efficiency at the same time. For example, the use of a vertical furnace suffers from substantial variations in furnace temperature distribution and the progress of nitriding reaction, resulting in silicon nitride powder having a largely varying content of .alpha. phase. The method using a pusher type tunnel furnace can produce silicon nitride powder having a relatively consistent content of .alpha. phase, but in commercially undesirable yields. A rotary kiln is difficult to control the residence time, and so, larger variations occur in .alpha. phase content and other quality factors and stable operation is unexpectable. The method using a fluidized bed can produce silicon nitride powder having a relatively consistent content of .alpha. phase, but long term furnace heating and cooling cycles and slow nitriding reaction impose some limitation to its commercial production rates.
More particularly, Japanese Patent Application Kokai No. 97110/1986 is directed to the preparation of silicon nitride powder by direct nitriding through a fluidized bed. Alpha phase rich silicon nitride powder is prepared by fluidizing metallic silicon powder with a reaction gas containing nitrogen or ammonia gas while the powder is heated at a controlled rate to prevent melting and agglomeration of silicon nitride powder. Due to batchwise operation, this method requires a long time for heating and cooling, resulting in commercially less desirable production rates.
The continuous preparation of silicon nitride powder by direct nitriding through a fluidized bed reactor has the following problems with respect to supply of metallic silicon powder to the reactor and withdrawal of silicon nitride powder from the reactor.
(i) The metallic silicon powder supplied to the reactor is exposed to high temperatures at which particles agglomerate and fuse so that metallic silicon powder is not constantly available at the reaction zone.
(ii) The metallic silicon powder supplied to the reactor is not uniformly dispersed so that contacting metallic silicon particles agglomerate and fuse together, failing to ensure the stable operation of the reactor and the consistent production of quality silicon nitride powder.
(iii) Silicon nitride powder is typically withdrawn from the reactor through a discharge duct in an overflow mode. As silicon nitride powder adheres to the duct wall, the duct passage becomes narrower and is eventually clogged.
The prior art methods suffer from ambivalence in that production rates are lowered if variations in quality factors like .alpha.-phase content are minimized to a satisfactory level, while quality control becomes difficult if production rates are increased to a commercially acceptable level.